1. Field of the Invention
The present invention relates to a workpiece gripping device for gripping a workpiece, and in particular, to a workpiece gripping device used for a robot to pick up individual workpieces from densely stacked workpieces or workpieces stacked in a box.
2. Description of the Related Art
In a manufacturing process, many workpieces can be stacked at a predetermined place or in a storage box, and it is known to use a robot to automate the picking of individual workpieces and supplying the workpieces to a subsequent process. In a picking job, it is necessary to avoid interference between the robot (specifically, a workpiece gripping device thereof) and an object (for example, other workpieces or a side wall of the storage box) surrounding the workpiece to be picked. In order to meet this requirement, Japanese Unexamined Patent Publication No. 2002-331480 discloses a technique for automatically judging whether or not interference occurs.
The robot for performing the picking job is also provided with a workpiece gripping device, and various types of object gripping devices are known. For example, Japanese Unexamined Patent Publication Nos. 2007-139360 and 2008-23664 propose a device which grips an object by sucking an upper surface of the object and then pressing against the side surfaces of the object so as to sandwich it from both lateral sides. Japanese Unexamined Patent Publication Nos. 2002-331488 and 11-20997 also propose a device which grips a workpiece by inserting a thin-plate member into between workpieces closely contacting each other and pressing on the workpiece so as to sandwich it from both lateral sides using the inserted member. Further, Japanese Unexamined Patent Publication No. 2004-25326 proposes a device in which one gripping device has a plurality of griping mechanisms mounted thereon and several types of workpieces can be picked using a gripping mechanism suitable for picking each type of workpiece.
As described above, the picking of individual workpieces from a stack of workpieces using the robot is performed while making sure that the workpiece gripping device does not interfere with an object surrounding the workpiece to be picked. When an interference occurs, an operator is required to interrupt the job in order to adjust the operation of the robot or seek a new job procedure causing no interference including a robot posture for workpiece picking. Therefore, in order to improve job efficiency, it is desirable to make it more difficult for interference to occur.
In the gripping devices disclosed in Japanese Unexamined Patent Publication Nos. 2007-139360 and 2008-23664, a member for gripping side surfaces of an object is provided on the lateral sides of a suction portion. Therefore, in the case where these gripping devices are used for picking a workpiece, a large space is necessary around the workpiece to be picked in order to prevent interference between the member for gripping the side surfaces of the workpiece and an object surrounding the workpiece. Especially for workpieces densely stored or located near the side surface of the storage box, no space can be secured around the workpieces and the workpieces are sometimes difficult to pick.
Further, in the gripping devices disclosed in Japanese Unexamined Patent Publication Nos. 2002-331488 and 11-20997, the thin plate member adapted to be inserted to between workpieces cannot be suitably inserted to between the workpiece closely contacting the wall and the wall of the storage box in which the workpieces are stacked. Therefore, it is sometimes difficult to pick such workpieces. Further, when inserting the member in between workpieces, the member inserted may cause measurable damage on the workpiece surface, depending on the type of the workpiece or the magnitude of the force exerted between the stacked workpieces. Therefore, the application of these gripping devices is limited.
The gripping device disclosed in Japanese Unexamined Patent Publication No. 2004-25326 has a plurality of gripping mechanisms and therefore is liable to become large. This also easily causes interference with an object surrounding the workpiece to be picked. Therefore, this gripping device is not suitable for carrying out a job in a limited space in a storage box or the like.